This kind of methods for the illumination of particles as well as the analysers utilizing such methods are widely known in industrial use. Today it is customary in the industries to transport or treat a material in the form of a particle suspension in a medium, such as a liquid. The particle may thereby be e.g. a crystal, fibre, grain, bubble, droplet, etc. The medium may be e.g. water or a suitable gaseous substance.
The observation and control of different kinds of manufacturing processes requires data on the amount, size distribution or shape of particles. It is of advantage if such data can be obtained quickly and continuously. Therefore, it is selfevident that the best way of obtaining the data would be to use an analyser which measures directly from a process pipe or a container. This would guarantee that the measured sample gives reliable information on the process.
According to the principle of optical measurement, particles are illuminated in a particle analyser, and the light reflected by the particles is measured by means of an optical detector. The choice of the optical detector depends on the analysis to be carried out in each particular case. The optical detector may thus be of any type, beginning from a simple photocell up to a multi-element image analyser. The reflected light can also be examined with the naked eye or by means of suitable auxiliary optics. Essential is that the illlumination of the particles is so arranged that the illuminated volume part of the medium is accurately defined and known. When only the particles contained in a determined volume part are illuminated, a good contrast is obtained for the image received by the optical detector. As to quantitative analysis, a decisive advantage is obtained when the liquid volume in which the illuminated particles are contained is known.
Amongst previous solutions may be mentioned a crystal microscope widely used in the sugar industries, e.g. Jungner Crystal Projector type KP3. This device comprises two windows between which the suspensions to be measured is passed. The windows form two mutually spaced parallel planar surfaces the spacing of which is adjustable. Particles, e.g. sugar crystals, are illuminated through one window, and the image of the particles is formed by means of an objective positioned behind the other window. The image is projected on a matt-finished glass plate on an enlarged scale so that the operators are able to observe the size and the shape of the particles. The contrast is poor, because the illuminated particles are seen against the light, so that no accurate electronic automated image analysis is possible. No quantitative conclusions, either, can be drawn, because the flow in the slit defined between the windows does not provide reliable information on the main flow, i.e. the sample is not representative enough.
Another very large group of known devices consists of so-called photometers which measure the absorption or the reflection of light caused by the particles. A typical example of photometers is the device disclosed in U.S. Pat. No. 3,962,581. It is characteristic of a device of the photometer type that the illumination is formed by a cone of rays passed substantially perpendicularly through the process window. One drawback is that the illuminated volume part is not accurately defined, do that the device can be used only for a quantitative determination based on an adjustement curve. The adjustment curve has to be determined experimentally separately for each application.
Another known photometer-type solution is the so-called Total Power method, which is part of the Nordmiljo 80 project. By means of this solution it is possible to obtain some qualitative information, too; mainly concerning the fibre length distribution. In the device applying the solution, the suspension flows within a glass tube and the device is used for measuring the average value and the time distribution of the light reflected by the particles. The ratio of the distribution and the average value serves as a kind of measure for the particle size. This solution, too, has the disadvantage that it cannot be used for a determination carried out directly from a process, because the sample flows in a small measuring cuvette. Accordingly, the representativeness of the sample is questionable.